1. Field of the Invention
Embodiments disclosed herein relate generally to fluid loss pills for use in oilfield operations.
2. Background Art
When drilling or completing wells in earth formations, various fluids typically are used in the well for a variety of reasons. The fluid often is water-based. For the purposes herein, such fluid will be referred to as “well fluid.” Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, minimizing fluid loss into the formation after the well has been drilled and during completion operations such as, for example, perforating the well, replacing a tool, attaching a screen to the end of the production tubulars, gravel-packing the well, or fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, fluid used for emplacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
However, for a wellbore fluid to perform all of its functions and allow wellbore operations to continue, the fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the wellbore fluid may be lost to the formation. For example, wellbore fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole.
When drilling progresses to the depth of penetrating a hydrocarbon bearing formation, special care may be required to maintain the stability of the wellbore. Examples of formations in which stability problems often arise include highly permeable and/or poorly consolidated formations. In these types of formations, a drilling technique known as “under-reaming” may be used. In under-reaming, the wellbore is drilled to penetrate the hydrocarbon bearing zone using conventional techniques. A casing generally is set in the wellbore to a point just above the hydrocarbon bearing zone. The hydrocarbon bearing zone then may be re-drilled, for example, using an expandable under-reamer that increases the diameter of the already-drilled wellbore below the casing.
The high permeability of the target formation, however, may allow large quantities of the drilling fluid to be lost into the formation. Once the drilling fluid is lost into the formation, it becomes difficult to remove. Calcium and zinc bromide brines, often used in completion fluids, can form highly stable, acid insoluble compounds when reacted with the formation or substances contained therein. This reaction may reduce the permeability and conductivity of the formation near the wellbore and thereby reduce the subsequent out-flow or production of targeted hydrocarbons. For example, fluid leakoff into the formation can occur during gravel placement and/or screen installation due to overbalance pressure, i.e., the difference in hydraulic head and reservoir pressure. After perforating, as another example, the completion fluid also tends to leak into the formation. The completion fluid can also be lost during the trip out and trip in to assemble the production tubing and the screen after the well is gravel packed. Generally, a flapper valve is used to isolate the screen and the formation from the wellbore fluids after the service tool and wash pipe are pulled out of the screen. If the flapper valve fails to isolate, the brine can be lost to the formation.
Providing effective fluid loss control without damaging formation permeability in completion operations has been a prime requirement for an ideal fluid loss-control pill. To control the fluid leak-off to the formation, a fluid loss control pill may be used to block the perforations or to form a filtercake on the formation face. In the case of fluid loss through the screen during trip out for assembling the screen and the production tubular, the fluid loss pill is spotted inside the screen to block the openings in the screen.
Conventional fluid loss control pills include oil-soluble resins, calcium carbonate, and graded salt fluid loss additives that have been used with varying degrees of fluid loss control. These pills achieve their fluid loss control from the presence of solvent-specific solids that rely on filter-cake build up on the face of the formation to inhibit flow into and through the formation. However, these additive materials can cause severe damage to near-wellbore areas after their application. This damage can significantly reduce production levels if the formation permeability is not restored to its original level.
After any completion operations have been accomplished, removal of filter cake formed from the fluid loss pill remaining on the sidewalls of the wellbore may be necessary. Although a fluid loss pill may be essential to completion operations, the barriers can be a significant impediment to the production of hydrocarbon or other fluids from the well if, for example, the rock formation is still plugged by the barrier. Because filter cake is compact, it often adheres strongly to the formation and may not be readily or completely flushed out of the formation by fluid action alone.
Accordingly, there exists a continuing need for fluid loss pills that may effectively reduce the ingress and egress of fluids between the formation and wellbore during a completion operation but that also minimizes formation damage.